Shield arrangement for ECG lead wires

ABSTRACT

The present invention discloses a grounding arrangement in a system using same connector for 5- and 12-lead connection in ECG-measurement. If 5-lead configuration is used, the connector elements of the 12-lead connection are used for grounding of lead wire shields. In the system according to the invention grounding of the lead wire shields is performed through current limiting element.

FIELD OF THE INVENTION

[0001] The present invention relates to a system for ECG monitoring. Inparticular, the present invention relates to a novel system and methodfor grounding of the shields of electrode lead wires.

BACKGROUND TO THE INVENTION

[0002] In prior art is known a system for ECG monitoring as representedby the diagram in FIG. 1. In FIG. 1, the same patient P isdiagrammatically represented by two pictures of the torso, where thelower picture shows a standard four-point placement of measuringelectrodes R, F, L and N (so-called limb electrodes). For the sake ofclarity, the upper picture separately shows a standard placement of ECGmeasuring electrodes, i.e. precordial electrodes V₁, V₂, V₃, V₄, V₅, V₆on the patient's thorax. Together the limb electrodes and precordialelectrodes form a so-called 12-lead connection system. A so-called5-lead connection system, also used in ECG measurement, consists of limbelectrodes together with one of the pre-cordial electrodes, e.g. R, F,L, N and V₁.

[0003] Each measuring electrode is provided with a signal lead wire 1-10which are further provided with first connector elements 11. The ECGmonitoring system further comprises a collecting connector 13 providedwith second connector elements 14 for receiving the first connectorelements 11 of the signal lead wire. The collecting connector may resideeither in the collecting cable 12 with an adapter 24 at its end ordirectly at the amplifier unit 16. The system further comprises an ECGapparatus 30.

[0004] The circuitry for measuring 5 or 12-lead ECG is presented in FIG.2. The collecting connector 13 comprises twenty connector elements 14 inall. Placed in the upper row on the left are circular connector elements141 for the upper part of connector elements 11 providing connectionsfor the signal lead wires 1-5 coming from the limb electrodes R, F, L,N, and one precordial electrode V₁. Connected to the circular connectorelements 14 in the right-hand part of the upper row are the upper partof connector elements 11 providing connections for the signal lead wires6-10 coming from the precordial electrodes V₂, V₃, V₄, V₅ and V₆.

[0005] The rectangular connector elements 142 in the lower row are forthe lower parts of connector elements 11 providing connections for thelead wire shields SH. Connector elements 14 ₂ provide a shielding groundconnection 39, which in this case is the amplifier ground G. The dualconnector elements 11 are consistent with the AAMI standard.

[0006] The function of the N-electrode is to equalize the potentials ofthe patient and the amplifier. For this purpose, the N-electrode istraditionally connected to the amplifier ground G either directly orthrough a single resistor or a network of resistors and capacitors. Thepotential difference may be further reduced with an electronic circuit34 actively driving the potential of the patient closer to the potentialof the amplifier, often referred to as right-led-drive circuit (RLD).This kind of driver includes a current limiting circuit typicallyadjusted to a maximum current allowed in a single-fault condition(EN60601-1).

[0007] Resistors R protect the preamplifiers 35, RLD-circuit 34, andimpedance measurement circuit 33 from over-voltages and limit thecurrents through electrodes in case of defibrillation.

[0008] Also in prior art, patent application WO 01/06923 discloses alead set and connector arrangement system as shown in FIG. 3, which canoperate both as a 5-lead measuring system and as a 12-lead measuringsystem, and in which it is possible to combine the limb-electrode andprecordial electrode parts of the collecting connector 13, allowing thesame amplifier unit to function alternatively in a 5-lead measuringsystem with shielded lead wires 37 or in a 12-lead measuring system withunshielded lead wires 38. In this system a small and lightweightcollecting connector can be used.

[0009] The circuitry for measuring 5 or 12-lead ECG is presented in FIG.4. In this arrangement, when measuring 12-lead ECG, the circular secondconnector elements 14 ₁ are used for the first connector elements 11 ofthe lead wires 1-5 coming from the limb electrodes R, F, L, N, and oneprecordial electrode V₁ (un-shielded 12-lead set 38). Connected to therectangular second connector elements 14 ₂ of the lower row are thefirst connector elements 11 of the lead wires 6-10 coming from theprecordial electrodes V₂, V₃, V₄, V₅ and V₆ (unshielded 12-lead set 38).

[0010] In case of 5-lead ECG one possible lead-set configurationconsists of AAMI-compatible connectors with shields connected to thelower, rectangular connectors 14 ₂ (shielded 5-lead set 37). In thiscase, analog switches 18-22 inside the amplifier unit may be used toprovide shielding ground connection 39 for the lower connector pins 14 ₂and hence for the lead wire shields SH.

[0011] Use of shielded signal lead wires has an importance especially inECG monitoring performed during anesthesia, because surgical operationsare often performed using a so-called diathermy device, i.e. an electricsurgical knife, whose high-frequency electric current would otherwiseconfuse the ECG monitoring process, but also in intensive caremonitoring, where line voltage interference and electrostatic voltagescoupling directly to the lead wires may be a significant problem.

[0012] Considering the prior art, there remain unresolved technicalproblems related to the use of the analog switches 18-22 for providingshielding ground connection for the lead wire shields. When changingover from 5 to 12-lead measuring mode by adding precordial electrodes, alow-impedance current path is created between the right-leg-drivecircuit 34 and amplifier ground G, if the precordial electrodes getconnected to the amplifier ground G by via analog switches 18-22.Because of small offset voltages always present in this kind ofelectronics circuit, the output of the RLD circuit may be driven to thecurrent limit, which makes the RLD circuit effectively inoperative.Furthermore, because connection of electrodes and lead wires in thismanner is a normal operating procedure, the current limit of the RLDcircuit has to be set to conform with the 10 μA maximum allowed currentduring normal operation (EN60601-1), thus limiting the efficiency of thecircuit compared to the typically used 50 μA limit.

[0013] Considering the prior art, there also remains an unresolvedtechnical problem relating to the changeover from 5- to 12-leadmeasuring mode in case of the combined collecting connector. When theECG electrodes are initially attached to the patient and the analogswitches 18-22 are open, the ECG device is capable of deducing thenumber of electrodes used by analyzing the signal characteristics fromthe different signal lines. Once the analog switches are closed, it isnot possible to detect addition of precordial electrodes based on signalcharacteristics, there thus being no way for the amplifier to detect theaddition of the precordial electrodes and this way to automaticallychange over from 5 to 12-lead measuring mode.

[0014] To mitigate the above-mentioned shortcoming, patent applicationWO 01/06923 discloses a method of detecting the addition of precordialelectrodes based on a special arrangement related to the shield of the Nelectrode lead wire, which corresponds to the V₆ signal lead wire. Thisarrangement is not optimal because in this case the moment of changefrom 5 to 12-lead measuring mode in the monitor depends on the order inwhich the precordial electrodes are connected to the patient.

[0015] One possible solution would be to use the activation of the RLDcircuit current limiting feature as an indicator of addition of aprecordial electrode. This solution is not desirable, since it would bebased on the relatively low 10 μA current limit setting. Furthermore,the connection of a precordial electrode would make the RLD circuittemporarily inoperable.

PURPOSE OF THE INVENTION

[0016] The purpose of the invention is to overcome the above describeddrawbacks related to the alternate use of connector elements in an ECGconnector either for ECG recording or lead wire shield grounding.

SUMMARY OF THE INVENTION

[0017] In accordance with the invention, analog switches are used tocouple the shields of the electrode lead wires not directly to theamplifier ground, but to an electronics circuit (shield groundingcircuit) which has output properties that provide sufficient groundingof the lead wire shields, limit the current during normal operation, andenable detection of the addition of precordial electrodes.

[0018] In the preferred embodiment of the invention the shield groundingcircuit is a constant voltage source with a current limiting feature. Incase one or multiple precordial electrodes are added to the monitoringsetup, this kind of circuit draws a constant DC current from the RLDcircuit, but does not make it inoperable. The addition of the precordialelectrode(s) can be detected either based on the current limitingfeature or by analysis of recorded ECG from the precordial electrode(s).

[0019] Furthermore, in one embodiment of the invention the addition ofthe precordial electrode(s) can be detected using a voltage comparisonand, thus comparing the voltage over current limiting resistor orcomponent to the predefined reference voltage. As soon as the voltageover the resistor exceeds the certain limit the addition can bedetected.

[0020] In one embodiment of the invention the grounding circuit consistsof one or more passive components, like simple resistors, or resistorsand capacitors to further tailor the frequency response of the circuit.The circuit may also include one or more non-linear components, such asspark gaps or diodes. The DC and AC impedance of the system has to belarge enough to keep the current within the safety limits under allconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are included to provide afurther understanding of the invention and constitute a part of thisspecification, illustrate embodiments of the invention and together withthe description help to explain the principles of the invention. In thedrawings:

[0022]FIG. 1 is a diagrammatic representation of prior-art system,

[0023]FIG. 2 is an example of the circuitry of the system presented inFIG. 1,

[0024]FIG. 3 is a diagrammatic representation of another prior-artsystem, which the present invention is related to,

[0025]FIG. 4 is one embodiment of the circuitry of the system presentedin FIG. 3 enhanced with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Reference will now be made in detail to the embodiments of thepresent invention. FIG. 4 represents a complete system in which a shieldgrounding circuit 27 according to the invention is applied to prior artsystem.

[0027] As shown in FIG. 4, the collecting connector 13 in the prior artembodiment contains a number of connector elements 14; 14 ₁, 14 ₂corresponding to the number of measuring electrodes defined in the12-lead ECG standard, i.e. a total of 10 connector elements. As can beseen, the connection elements for the lead wires 1-4 connected to thelimb electrodes R, F, L, N and for one lead wire 5 of the precordialelectrode V₁ are arranged in the upper first row 14 ₁ in the collectingconnector 13. The connection elements for the rest of the lead wires6-10 connected to the rest of the precordial electrodes V₂, V₃, V₄, V₅,V₆ are arranged in the lower second row 14 ₂ in the collecting connector13 the second row 14 ₂ being in paired alignment with said first row.

[0028] The electronics of the amplifier unit in FIG. 4 is provided withanalog switches 18-22 controllable by the microprocessor of the ECGapparatus (not shown). If the analog switches 18-22 are in “open”connection positions, the measurement signals are passed from all themeasuring electrodes R, F, L, N; V₁, V₂, V₃, V₄, V₅, V₆ in the 12-leadset 38 to the preamplifiers 35 and further to the signal processingstage in the ECG apparatus (not shown). In the “closed” connectionpositions the analog switches 18-22 establish a connection between thesecond group of connector elements 142 in the lower second row of thecollecting connector 13 and the shield grounding circuit 27. When theanalog switches 18-22 are in the “closed” connection position, thesecond group of connector elements 142 can be used for providingshielding ground connections for possible electrostatic shield over the5-lead system 37 lead wires 1-5 connected to the upper first row 141.

[0029] With this arrangement, both 5 and 12-lead ECG measuring modes canbe used with the same compact collecting connector which resembles aconventional existing collecting connector designed for a 5-lead system,which means in practice that the shielding ground connections 142 of the5-lead system 37 are utilized by the precordial electrodes V₂, V₃, V₄,Vs, V₆ of the 12-lead system 38 as signal lines. In the 12-leadmeasuring mode, the signal lead wires are used without shielding groundconnections, whereas in the 5-lead measuring mode the existing signallead wires are used with shielding ground connections as usual.

[0030] Block 27 in FIG. 4 illustrates the preferred embodiment of theinvention, in which shield ground connection 39 is directed to theshield grounding circuit block 27.

[0031] The electronics in the shield grounding circuit 27 is builtaround an operation amplifier 28, the circuit operating as a voltagebuffer. The output impedance of this buffer shall be relatively low,e.g. less than 10 kohm, preferably over the whole frequency range of theECG signal, e.g. 150 Hz, or at least over the line voltage frequency,i.e. 50 Hz, as long as the current at the output of the shield groundingcircuit is below a predefined limit, e.g. 10 μA. If current in theoutput exceeds this limit, the output impedance of the circuit goes veryhigh, thus limiting the current to the predefined value.

[0032] In block 27 resistor 29 acts as a current limiting component.During normal operation electric fields coupling to the lead wireshields generate currents that are significantly smaller than the abovementioned current limit, the amplifier thus remaining in low outputimpedance mode.

[0033] The reference voltage to this buffer is DC voltage 31. This DCvoltage is selected to be slightly different from the target value ofthe RLD-circuit. It may even be nominally the same, but in practice alsothis results to the desired behavior, because of non-idealities likeoffset voltages in the amplifier components. Initially, the potential ofthe patient in respect to the amplifier ground is set to target value ofthe RLD circuit. When at least one precordial electrode is added, a DCcurrent flows from the RLD to the shield grounding circuit. This currentis limited to the value determined by the shield grounding circuit 27.If this current limit (typically 10 μA) is lower than the current limitin the RLD circuit (typically 50 μA), the ECG measurement is notdisturbed by these events.

[0034] The addition of a precordial electrode can be easily detected byobserving the state of current limit of the shield grounding circuit,which is activated. Considering the circuit in block 27, this isperformed by monitoring the output voltage of amplifier 28, which isdriven to either upper or lower rail. The monitoring can be done usingthe voltage comparator 32 that will give an output signal indicative ofthe exceeding of the reference voltage. This signal is led to the ECGmonitoring device (not shown). This detection mechanism is sensitive toa change in any of the precordial lead wires, thus enabling the monitorto change mode immediately independent of the order in which theprecordial electrodes are attached to the patient.

[0035] The invention is not restricted to the above represented examplebut the other applications of the invention according to the inventivestep of the patent claims are possible.

1. A grounding arrangement in a system for ECG monitoring in which someof the connector elements (14) of the lead set connector (13) arealternatively used either for recording of ECG signals or for groundingof lead wire shields, characterized in that the grounding is performedthrough a current limiting circuit (27).
 2. A grounding arrangement asdefined in claim 1, characterized in that the current limiting circuit(27) exhibits non-linear voltage-current characteristics.
 3. A groundingarrangement as defined in claim 1, characterized in that the currentlimiting circuit includes a current-limited voltage source.
 4. Agrounding arrangement as defined in claim 3, characterized in that theactivation of the current limiting function is used for detection of theaddition of new measuring electrodes.
 5. A grounding arrangement asdefined in claim 1, characterized in that the current limiting circuitconsists of passive components, at least one of which exhibitsnon-linear voltage/current characteristics.
 6. A grounding arrangementas defined in claims 1, 2, 3, 4 or 5, characterized in that there areindividual current limiting circuits for each connector element.
 7. Agrounding arrangement as defined in claim 1, 2, 3, 4 or 5, characterizedin that one single current limiting circuit is used for more than oneconnector element.
 8. A grounding arrangement as defined in claim 1, 2,3, 4 or 5, characterized in that one single current limiting circuit isused for all connector elements.
 9. A grounding arrangement as definedin claim 4, characterized in that the current limiting circuit (27)includes a detecting circuit (32) for detection of the addition of newmeasuring electrodes.
 10. A grounding arrangement as defined in claim 9,characterized in that the detecting circuit (32) is a comparator.
 11. Agrounding arrangement as defined in claim 9, characterized in that thedetecting circuit (32) is a analog-to-digital converter.
 12. Method forgrounding in a system for ECG monitoring in which some of the connectorelements (14) of the lead set connector (13) are alternatively usedeither for recording of ECG signals or for grounding of lead wireshields, characterized in that the lead wire shields are groundedthrough a current limiting circuit (27).
 13. Method as defined in claim12, characterized in that said current limiting circuit (27) is arrangedto exhibit non-linear voltage-current characteristics.
 14. Method asdefined in claim 12, characterized in that the addition of new measuringelectrode is detected through a current limiting circuit (27). 15.Method as defined in claim 12, characterized in that the method furthercomprises the steps of: measuring a current coming to said currentlimiting circuit, and when the measured current exceeds a predefinedlimit current, detecting the addition of new measuring electrode.